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THE 


FLORENCE OIL-FIELD, 


COLORADO. 


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(JEO. H. ELDRIDGE, 

*» 

U. S. GEOLOGICAL SURVEY, WASHINGTON, D. C. 



A Paper read before the American Institute of Mining Engineers, 

Glen Summit Meeting, October, 1891. 



AUTHOR’S EDITION. 


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1891 . 



















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[TRANSACTIONS OF THE AMERICAN INSTITUTE OF MINING ENGINEERS.] 


« 


THE FLORENCE OIL-FIELD , COLORADO. 

BV GEO. H. ELDRIDGE, U. S. GEOLOGICAL SURVEY", WASHINGTON, D. C. 

* (Glen Summit Meeting, October, 1891.) 

* 

Introduction. 

This sketch of the Florence oil-field, presented to the Institute by 
. permission of the Director of the U. S. Geological Survey, is accom¬ 
panied with a map, Fig. 1, the topography of which has been taken 
from a field-manuscript of the Survey. The scale of the map and 
the sections, Fig. 2, as herewith printed, is 3 miles to the inch. The 
symbols employed to indicate geological formations, etc., are: 

a, Archlean ; S, Silurian; C, Carboniferous; T, Trias; J, Jura; 
I\d, Dakota (Cretaceous); Kb, Benton; Kn, Niobrara; M, Mon¬ 
tana (including Pierre and Fox Hills); Kl, Laramie; q, Quarter¬ 
nary; O, Oil-zone; W, Wells. 

The dots in the vicinity of the town of Florence indicate the 
positions of the wells. 

The outlines of the Tertiary formations have been omitted inten¬ 
tionally. The areas of these formations are very small. 

Geographical Position of the Field .—The Florence oil-field lies in 
Fremont Co., Colorado, along the valley of the Arkansas river in 
close proximity to the eastern base of the Rocky Mountains. Its 
present center of production is near the town of Florence, 92 miles 
a little west of south from Denver, 30 miles in a west-northwest 
direction from Pueblo, and 8 miles below Canon City—the point of 
exit of the river from the range. Its rail communication east and 
west is by the Denver and Rio Grande, and Atchison, Topeka and 
Santa Fe systems. 

Development of the Petroleum Industry .*—Indications of petroleum 
were first observed in the course of early explorations by an army 
officer in the cafion of Oil creek, at a point about six miles northeast 

* For much of the earlier history of development, acknowledgment is due Prof. 
M. 0. Ihlseng’s Report on Oil-Fields of Fremont County , in the Annual Report of 
the Colorado State School of Mines for 1885. 









2 


THE FLORENCE OIL-FIELD, COLORADO. 


Fig. 1. 



Map of the Florence Oil-Field, Colorado. 

















































































LONGITUDINAL SECTION OF FLORENCE OIL FIELD 


THE FLORENCE OIL-FIELD, COLORADO 


3 































































4 


THE FLORENCE OIL-FIELD, COLORADO. 


of the present site of Canon City. The occurrence was in the form of 
seepage in the creek-bottom, at the foot of bluffs of Jurassic age. 
It was in this locality, in March, 1862, that the first active pros¬ 
pecting was begun by a Mr. Cassidy, who sank six wells to a depth 
of from 60 to ffO feet-and two to a depth of 400 feet, but found oil 
only in the strata near the surface. This was pumped, crudely re¬ 
fined, and shipped to Denver and other territorial markets, where it 
sold for $1.25 to $2.85 per gallon, the price at one time reaching $5 
per gallon. The sales up to 1870 are said to have amounted to 
3000 gallons. 

In 1866 and 1873 two other wells were sunk to depths of 312 
feet and 342 feet respectively, without developing any other than 
the supply already encountered a short distance beneath the surface. 

Subsequently to this—1877 to 1885—three companies, the Ar¬ 
kansas Valley Oil and Land Company, commonly known as the Cas¬ 
sidy Company, the Land Investment and Coal and Iron Company, 
afterwards becoming through one or two transitions the Colorado 
Oil Company, and the Canon City Oil Company, undertook explora¬ 
tions on the prairie bordering the Arkansas river below Canon City. 
These were the first explorations in strata of the Montana group of 
the Cretaceous, the group that alone affords the yield of to-day. 

The Arkansas Valley Company was developed from individual 
interests which embraced two wells: one experimental, located 
a half mile below Canon City, to the south of the river, bored 
in 1877 and abandoned at a depth of 900 feet without discover¬ 
ing oil; the other near the present town of Coal Creek, sunk 
in 1880 for water, in which oil was encountered at a depth of 1200 
feet, but which was abandoned on account of loss of tools and liti¬ 
gation. The company wells were located about If miles south- 
southeast of the present site of Florence, were 6 (7?) in number, 
ranged in depth to 1740 feet (one to 3012 feet), one of which was 
dry, the others producing from J barrel to 20 barrels (one, accord¬ 
ing to account, 100 barrels per day), some of which are yielding at 
the present time. 

The Land Investment and Coal and Iron Company was incorpo¬ 
rated in 1882, subsequently dissolved, was taken up by D. G. Pea¬ 
body its former president, and finally became the Colorado Oil Com¬ 
pany of 1885-6. The wells of this company in the latter year num¬ 
bered twelve, were with one exception within } mile of the site of 
Florence, ranged in depth to 1700 feet, and included several good 
producers, although the majority were small in yield. This com- 



o 


THE FLORENCE OIL-FIELD, COLORADO. 

pany and the Arkansas Valley Company were in the year 1887 
absorbed by the United Oil Company of the present day. 

The Cailon City Oil Company was incorporated in May, 1883. 
Tts single well is located in the vicinity of Oil creek about 3 miles 
east-northeast of Canon City, and 6^ miles northwest of Florence. 
Oil is reported as having been encountered at a depth of 300 feet, 
followed at a short interval by salt water which eventually caused 
the abandonment of the hole; a depth of 1700 feet, however, is said 
to have first been reached. 

The Companies at present in the field are, in order of magnitude: 
the United, the Florence, the Rocky Mountain, and the Triumph, 
the first two having been established about the year 1887, the others 
more recently, the Rockv Mountain Company being the youngest 
(1890). 

In addition to the wells of these companies, there are distributed 
over the field at considerable distances from the present center of 
production, a number of holes that have been drilled with varying 
success by individual enterprise. 

Topography. 

The Florence oil-field is confined within the limits of a synclinal 
basin. It is elliptical in area, the major axis having a trend of N. 
15° W. and a length of about 14 miles, the minor, a length of 8 
miles. Its topographical features are about equally attributable to 
structure, erosion, and rock-textures. The eastern rim of the basin, 
never so completely developed as the other portions of the periphery, 
has been considerably abraded by erosion. This is especially the 
case north of the Arkansas river. To the south of the river, how¬ 
ever, the rim is still clearly visible in the gentle rise of land to the 
southeast, its exterior edge being especially accented by a sharp drop 
in contours at the locally eastern limit of the Montana group. South¬ 
ward this rim passes directly into a minor though well developed 
ridge that juts at an acute angle from the main range of mountains 
forming the western boundary of the synclinal. Northward the 
eastern rim passes into what might at first be regarded as the west¬ 
ern spur of a double echelon-fold, the lofty mass of mountains in¬ 
volved in the flexures carrying the face of the range fully twenty- 
five miles to the east, and presenting the finest illustration of the 
echelon type of topography and its manner of development, to be 
found in the entire length of the Colorado range. 


6 


THE FLORENCE OIL-FIELD, COLORADO. 


In the western half of the field and south of the Arkansas river 
lies a conspicuous mesa, co-extensive with the Laramie formation, 
which is here composed of a succession of heavy bedded sandstones 
that on the northern, eastern, and southern periphery of the mesa 
form abrupt bluffs overlooking the rolling prairies of the Montana. 
There is also an outlier of the Laramie to the north of the river, 
two and a half miles northwest of Florence, known as “Castle Rock.” 

Erosion together with preservation beneath caps of Quaternary 
gravel has given existence to several other prominent heights through¬ 
out the field but in none of them are found strata of younger age 
than the Montana. 

The drainage of the area is by the Arkansas river and its lateral 
tributaries, the latter carrying but slight surface flows. 

Geology. 

Description of the Formations. 

The stratigraphy of the Florence oil-field and its confines embraces 
formations of Archaean, Silurian, Carboniferous, Triassic, Jurassic, 
Cretaceous, and Tertiary ages. Of these the Cretaceous formations 
are of chief concern in the present study; they comprise the Dakota, 
the Benton and Niobrara (together constituting the Colorado group), 
Pierre and Fox Hills (constituting the Montana group), and the 
Laramie. 

On the northern and northern third of the western edgre of the 
field the sedimentary formations from the Colorado down lie in a 
fringe along the Archaean and dip quite uniformly 30° beneath the 
prairie; a short distance south of the Arkansas river, however, 
structural peculiarities assert themselves, and from this point south¬ 
ward to beyond the limits of the field the Archaean is in direct 
contact with the Pierre shales of the Cretaceous. The formations 
younger than the Colorado lie wholly within the prairie region. 

The Older Formations; the Laramie and the Tertiary. —The 
Archaean, consists of a series of granites and schists ; the Silurian 
and Carboniferous, of limestones; the Trias, of the well-known 
“ Red Beds ” of the Rocky Mountain region, a series of massive beds 
of coarse conglomerates and sandstones in its lower division, of finer 
brick-red sandstones, shales, and interspersed limestones in its upper ; 
and the Jura, of shales, limestones, and occasional sandstones. Pass¬ 
ing for the present directly to the Laramie, this formation comprises 
a series of heavy bedded sandstones with intercalated shales, and in 


THE FLORENCE OIL-FIELD, COLORADO. 


7 


the lower portion workable beds of excellent coal. In many parts 
of Colorado the proportion of shales to sandstones is much greater 
than here. J he Tertiary formations are two; the lower, a series of 
coarse conglomerates and sandstones derived from the older forma¬ 
tions, 250 feet to 300 feet in thickness; the upper, a succession of 
conglomerates, sandstones, and shales of eruptive debris. The former 
probably corresponds to the Arapahoe formation of the Denver field, 
the latter to the Denver formation of the same region. The areas 
occupied by the two Tertiaries are small and isolated. 

The Cretaceous formations have the same general characteristics 
in the present field as elsewhere in the Rocky Mountain and prairie 
regions. 

The Dakota Formation. —This, the basal member of the series, is 
essentially a sandstone formation between 200 and 300 feet thick, 
divided into two or more benches, by bands of fire clay or other 
argillaceous matter, the lowest bench having a prominent and per¬ 
sistent zone of characteristic conglomerate at its base. The material 
of the sandstones is a white quartz, of fine to medium grain ; it is in 
many localites infiltrated with a siliceous cement which imparts to 
the rock a quartzitic appearance. The usual color of the Dakota 
sandstone is white or grayish-white, but it may be darkened several 
shades by the presence of iron or disseminated asphalt. This latter 
feature is not of infrequent occurrence in the West, and in several 
localities tlie bituminous matter seeps from the rock in well developed 
springs. In the Florence field the formation is heavily charged with 
mineral water which rises to the surface in copious flows wherever 
the strata are tapped by drilling. 

The fossils of the Dakota in Colorado are chiefly confined to plant 
remains. 

The Benton Formation .—This is a formation of black argillaceous 
shales, passing by transitional beds into the Dakota below, and the 
Niobrara above, though into the former in a manner more or less 
abrupt. Its thickness, like that of the other marine Cretaceous forma¬ 
tions in the West, varies considerably from point to point, but is 
generally between 400 feet and 800 feet, in the present field falling 
somewhat below the lower figure. In addition to its leading char¬ 
acteristics—its black or leaden-hiied color, argillaceous composition, 
and shaly nature—it is marked by the occurrence, here and there, 
of a few narrow and intermittent bands of fossiliferous limestones, 
more or less bituminous; by the frequent presence at certain hori¬ 
zons of concretionary, clay ironstones; and by the occasional appear- 


8 


THE FLORENCE OIL-FIELD, COLORADO. 

ance of a few narrow bands of impure argillaceous sandstones. As the 
summit of the formation is approached, the clays become more calcare¬ 
ous, and the limestone more prominently developed, the latter often 
bearing a distinct resemblance to the lower layers of the important 
bed at the base of the Niobrara, though rarely attaining a thickness 
of over four or five feet. Among the characteristic fossifs of this 
formation, Ostrea congesta and Inoceramus problematicus , together 
with Selachian teeth are especially well represented, the last parti¬ 
cularly in the more bituminous layers. 

The Niobrara Formation. —The thickness of this formation varies 
throughout the West from a mere trace to approximately 400 feet, 
approaching the latter figure in the present region. When fully 
developed it is separable into three divisions: the lowest, a bluish- 
gray, hard, coarsely laminated limestone of an average thickness of 
40 feet; the middle, a succession of gray marly clays, 100 feet; the 
uppermost, a series of calcareo-arenaceous clays of various shades of 
yellow or buff, remarkable for the presence of “ alkali salts ” and con¬ 
taining several bands of impure, yellow, saccharoidal, fossiliferous 
limestone. The basal member, from its general persistency and the 
prominence of its characteristics, forms an admirable datum-level 
for reference in the study of Cretaceous stratigraphical geology. 
The life of the Niobrara includes three especially prominent mollus- 
can forms, Ostrea congesta , Inoceramus problematicus , and Inocera¬ 
mus deformis , the last a very characteristic species ; fish integuments 
are also common. 

The Pierre Formation. —This formation is, in the main, a great 
body of leaden-gray clays, carrying from bottom to top lenticular 
bodies of impure limestone, and, in several localities, midway in the 
series of beds, a zone of yellowish, quartzose, and more or less 
calcareous sandstone of a thickness varying from 100 feet to 200 
feet. In the Florence field no evidence of this sandy zone exists, but 
at several points in the vertical scale there can be noticed a tendency 
of the clays toward a fine, arenaceous composition. 

Though the formation attains the exceptional thickness of over 
7000 feet in the vicinity of Denver, Cqlorado, it generally falls far 
below this figure, rarely attaining 1500 feet, and frequently only 
700 feet or 800 feet. In the Florence field the thickness is probably 
about 4450 feet. The clays are in general of remarkably uniform 
texture; often show a tendency to concretionary structure, though 
always clearly and evenly stratified; are plastic in a noticeable 
degree, and have a minor distribution of lime, gypsum and “alkali 


9 


THE FLORENCE OIL-FIELD, COLORADO. 

salts’ throughout. The lenticular bodies of limestone occur pro¬ 
miscuously through the formation and carry the bulk of its fossils. 
The dimensions of these bodies vary between 2 and 6 feet in the 
direction parallel to the bedding planes of the clays and between 6 
inches and 2 feet in the direction normal to these planes. Their 
composition lies between that of a clay with very little carbonate of 
lime, and a very pure limestone, generally inclining to the more 
calcareous variety; they are frequently reticulated with narrow cal- 
cite seams. In many of the limestone bodies, and occasionally in 
the clays, minute particles of carbonized vegetable tissues abound. 
The life of the formation is abundant, but its especially prominent 
forms are mainly included in the genera Inoceramus, Cucullcea , 
Nautilus , Placenticeras , Baculites , and Scaphites. As the summit 
of the Pierre is approached the clays very gradually become more 
and more arenaceous, though never losing their distinctly argilla¬ 
ceous composition until the indefinite line between this and the over- 
lying formation is passed, when the material becomes as marked an 
argillaceous sandy shale as it was before the reverse. 

The Fox Hills Formation .—Excepting its persistent and distinc¬ 
tive cap of yellow fossiliferous sandstones of a thickness of from 10 
to 50 feet, this formation embraces a series of fine arenaceous shales 
with a large percentage of clay, varying in thickness between 300 feet 
and 1000 feet, according to locality. In the Florence field their thick¬ 
ness has been estimated at 450 feet. Limestone concretions similar 
to those in the Pierre occur here and there throughout the shales. 
There is a transition in life as well as in sediment from the Pierre 
to the Fox Hills, and only at the summit of the latter formation 
does it become completely differentiated. 

The Thickness of the Montana Group (Pierre and Fox Hills) in the 
Florence Oil-field. —This has been taken at 4900 feet, of which 450 
feet have been allotted the Fox Hills, leaving 4450 feet as the thick¬ 
ness of the Pierre. The total thickness of the group, 4900 feet, is 
based upon the following evidence, an actual measurement on a defi¬ 
nite line being found impracticable. In one of the wells in the east¬ 
ern portion of the area at present developed, Pierre shales were alone 
passed to a depth of 3300 feet. The line of union of Pierre and Fox 
Hills indisputably lies to the west of the point at which the hole 
was drilled, and had the Niobrara been entered it would have im¬ 
mediately been recognized by its color and difference in chemical 
and physical composition from the Pierre. The base of the forma¬ 
tion was arbitrarily established 200 feet beneath the bottom of* this 


10 


THE FLORENCE OIL-FIELD, COLORADO. 


well. This additional distance in depth was taken : first, because 
no evidence existed of the base of the formation having been reached 
in the well, and it was therefore necessary that some increase should 
be made; secondly, 1J miles east of the well in question are two 
others, yielding artesian watef, the start in drilling them having 
been made at a horizon not over 50 feet beneath the summit of the 
Niobrara; thirdly, no dips were anywhere observed of greater 
degree than sufficient to carry the strata into the positions assigned 
them upon the above basis. The observed surface dips were usually 
much less than required under the above conditions, but flexures in 
the strata unquestionably occur to compensate any lack in this 
direction. At several points in the field such flexures were apparent. 
The thickness of the remainder of the Montana from the above well 
westward to the clearly defined line of the base of the Laramie is 
determined from careful observations of dip, distance, and difference 
in altitude. 

It is rarely that the Montana group can be separated into its com¬ 
ponent formations at any distinct line, but there is almost always a 
change of aspect both in its composition and its fossils within a zone 
of 100 feet or 200 feet that enables one to decide as to the presence 
of the one or the other. Such is the case in the present field, and 
at several points on the higher lands, in the vicinity of a line pass¬ 
ing N. 15 W. a half mile west of Florence, the rocks have a distinct 
Fox Hills aspect. On the visible dips (mostly 5°) between this line 
and that of the base of the Laramie to the west, with the requisite 
allowance for difference in altitude between the two, 450 feet has 
been regarded as the nearest possible approximation to correctness. 



between that determined for the Montana as a whole and that of the 
Fox Hills, 

Mr. Johnson, Superintendent of* the Florence Oil Company, has 
determined a thickness of 4500 feet for the Montana group, based 
upon measurements made by him in the foot-hills. 

The Structure of the Field. 

The Florence oil-field lies within a synclinal of elliptical area, the 
major axis of which has a trend of N. 15 W. and passes somewhat 
east of the center of the region occupied by the Laramie, the minor 
axis passing across the field about a mile south of the town of Coal 
Creek. The structure of the field and the related region that adjoins 


THE FLORENCE OIL-FIELD, COLORADO. 


11 


it on the east, has been brought about by the combined influence of 
what is in reality a comparatively slightly developed secondary 
mountain fold and a fold en echelon , the latter lying to the east of 
the former. 

The secondary fold appears in the anticlinal arch crossing the 
Arkansas river at the mouth of Brush creek, its axis passing just 
east of Kendrick mountain. Its southern limit is sharply emphasized 
in a fold jutting to the northward, at an acute angle from the main 
front range, at a point about thirteen miles due south of Florence. 
Northward the arch passes into what might be at first regarded as 
the western spur of a double echelon fold, but which is, however, 
distinct in structure, though in a measure related to, and probably 
of synchronous development with the true echelon fold to the east. 

The fold en echelon which lies wholly to the east of the area 
mapped for this paper, but which may be referred to on sheet XIII 
of Hayden’s Atlas of Colorado , has had an extensive influence upon 
the superficial relations of the Cretaceous formations east of the sec¬ 
ondary fold just described. Its axis lies well out on the prairie, and 
the effect of its arch, rapidly flattening as distance south is gained, is 
manifest in the outcrops of one or another of the lower three forma¬ 
tions of the Cretaceous, noPonly along the river from the eastern 
rim of the oil field to a point several miles below Pueblo, but also 
for a considerable distance to the south of the river. From within 
this great area of its influence the Montana group has for the most 
part been removed, and with it the oil-zone of the Florence field. 

Questions as to the presence of faults and fissures within the limits 
of the Florence oil-field have repeatedly arisen. That slight local 
displacements of strata have taken place can hardly be doubted, es¬ 
pecially in the shales of the Montana, which show numerous flexures, 
but the occurrence of faults of considerable displacement, affecting 
the series of formations underlying the field, is negatived by the 
enormous flows of artesian water from the Dakota sandstones wher- 
ever pierced. The points of entrance of such waters are at the out¬ 
crops of the formation at the base of the mountains, and their con¬ 
stant, and copious discharge precludes all possibility of interruption. 
Again, such slight displacements in the body of shales as may have 
occurred, no matter how widely distributed, can hardly have left the 
series open-fissured in such a manner, for instance, as to form reser¬ 
voirs for the reception of infiltrating oil, a view held by some, for 
the plasticity and yielding nature of the shales would, on account of 
accompanying pressure, very generally prevent the formation of such 


12 


THE FLORENCE OIL-FIELD, COLORADO. 


cavities. The possibility, however, of the occasional existence of open 
fissures within the Pierre is not denied, but they must be small and 
of slight extent both in vertical and horizontal range. Their exist¬ 
ence cannot a fleet the general views expressed beyond in regard to 
the nature of the occurrence of the oil, for in any event the fissures 
would be but reservoirs of a more open condition than the arenaceous 
bodies already containing the oil—a large pore in the formation— 
into which the oil would filter as into the sands. Upon establishing 
communication with the surface, they would become merely a part 
of the bore which pierced them. Of the existence, however, of such 
fissures, and encountering them in drilling, there is no authentic 
evidence whatever. 

The Horizon of the Oil. 

Lithological Character .—The distribution of the materials of the 
Pierre and its included oil-bearing zone, both in lateral and vertical 
range, is an important consideration in reference to the source and 
storage of the contained oil. The formation is chiefly one of argil¬ 
laceous shales, with occasional local developments of sandy strata of 
various degrees of coarseness. In the Florence field the coarser va¬ 
rieties of sand are wanting, but there often exists an extremely fine 
arenaceous matter intimately mixed with the clay. The deposition 
of these arenaceous materials has been either as a succession of beds 
passing laterally by more or less rapid gradations into clays, or in 
the form of irregular lenticular bodies of widely varying vertical and 
horizontal extent in the midst of the argillaceous layers. No regu¬ 
larity whatever exists in the position these bodies occupy in either 
the vertical or lateral range of the formation ; they may or may not 
have intercommunication ; they may be encountered at any point 
between base and summit of the shales; they frequently overlap. 

The distribution of the materials here odven for the oil-bearino’ 

o O 

zone of the Pierre, established both from well records and surface 
observations, is in harmony with nearly all the facts of occurrence 
of the oil, whether it is derived from several horizons in a single 
well or from but one point in depth, whether the yield be great or 
small at the same or different horizons, whether copious at one well, 
and at another within a comparatively short distance, the merest trace. 
In all, the yield is said to be from the more sandy variety of shale. 

Thickness of the Oil-Bearing Zone .—This is not yet established. 
There are but few borings of a greater depth than 2000 feet below 
the surface. In two or three of these, small yields have been en- 


THE FLORENCE OIL-FIELD, COLORADO. 


13 


countered, but the strata below the horizon assumed in the sections 
as the base of the zone, are still unexplored. Both the base and the 
summit of the zone in the sections have been graphically determined 
in accordance with the depths and locations of the shallowest and 
deepest producing horizons in any of the wells of the field. The 
thickness thus developed is 1947 feet. 

The position of the zone thus defined in the stratigraphica! series 
of the Montana beds it is impossible to determine absolutely, for, 
although in the Florence field the succession of strata from the sum¬ 
mit of the series down is very probably normal, when the entire area 
underlain by the series throughout the West is considered, the thick¬ 
ness of the group undergoes the greatest variation from point to point. 

Extent of the Field .—It is obvious from the foregoing that the 
periphery of the productive portion of the Florence field may lie but 
a short distance within the area of outcrop of the oil-bearing zone as 
established in the sections. There is no geological evidence against 
such possibility; there are arguments in its favor, both from the 
known composition and distribution of the materials in the Pierre 
and from the structure within the region; there is also direct evi¬ 
dence favoring the extension suggested in the presence of oil in several 
wells drilled at various points considerably beyond the outskirts of 
the present field of development. As regards direction, extension to 
the east will be slight on account of the proximity of the area already 
producing to the eastern rim of the synclinal; to the west it will be 
limited by questions of economy and depth to which drilling can be 
advantageously carried; while to the north and south obstacles to 
extension within the limits defined are apparently wholly absent. 
In the extension of the field to the west and south, but particularly 
in the former direction, consideration should be given to the character 
of the oil-bearing bodies, that is, that they are masses of arenaceous 
materials inclosed in clay. Dip will therefore have no effect upon 
the flow of the oil beyond the body immediately affording the yield, 
unless two such bodies should be intercommunicating through the 
character of their materials. 

The Source of the Oil —The lowest geological horizon at which 
oil has been observed in Colorado, is near the base of the Jurassic 
shales. Although the wells affording this yield were sunk in the 
gravels of the Oil creek bottom, it is the statement of Mr. Johnson, 
of the Florence Oil Company, that they were derived from sandy 
beds in the shales themselves. The yield is reported as having been 
very small, 1 barrel per well per day. 


14 


THE FLORENCE OIL-FIELD, COLORADO. 


Asphalt occurs in the Dakota in several localities in the West. 
There is an artificial spring at the base of the ridge near Turkey 
creek cation, 15 miles southwest of Denver. The product may here 
be seen slowly oozing from the bedding planes of the sandstones in 
the upper portion of the formation. For a half mile along this por¬ 
tion of the hogback, the sandstones are discolored a dirty brownish 
black, and when broken yield with a greater or less degree of flexi¬ 
bility. In many localities in Wyoming the Dakota is pre-eminently 
an oil bearing formation, though as yet undeveloped. 

The limestones and calcareous shales of the upper half of the 
Benton formation are quite generally noted for the strong odor of 
petroleum that comes from freshly fractured faces or upon rubbing 
two pieces together. Slips, faces, lamination planes, and the con¬ 
tained casts of animal remains, particularly sharks’ teeth, frequently 
have a coating of a bright, oily lustre. The beds possessing these 
characteristics are much blacker than the other portions of the for¬ 
mation. 

The Niobrara only occasionally yields the odor of petroleum in a 
few of its beds, chiefly the basal limestones. 

The Pierre has proved itself, at least for the Florence field, the 
great oil-producing formation. The oil cannot have been derived 
from the Jura, since the absorbent Dakota sandstones intervene; nor 
from the Dakota, since a coarse and porous sandstone would not 
yield any contained liquid to a body of shales ; nor from the Benton, 
since the lower limestones of the Niobrara are too non-absorbent. 
It is, moreover, difficult to conceive a system of Assuring sufficiently 
extensive to have permitted the passage of oil from lower beds to the 
Pierre over such an areal extent, even upon the supposition of a more 
absorbent composition of the latter formation, and the fact that the 
synclinal structure would have been conducive to Assuring from 
below upward. The possibility of Assures both of an •extent suffi¬ 
cient to have established communication between the formations below 
and the Pierre, and within the Pierre itself, has already been con¬ 
sidered ; but in this connection it may be further remarked that As¬ 
suring of the nature here suggested is beyond conception, for it could 
not have occurred in the Benton and overlying formations without 
occurring in the Dakota, in which case the great volume of water of 
the latter formation would have followed the oil and, upon exhaus¬ 
tion of this, appeared in the wells as an artesian flow. Not only is 
this not the case, but the majority of the wells remain dry after 
exhaustion, and none shows a water charged with salts peculiar to 


THE FLORENCE OIL-FIELD, COLORADO. 


15 


recognized Dakota flows. Moreover, the tendency of the Pierre 
shales, from their argillaceous composition, to close such fissures, 
has also been noted. 

It would seem probable, therefore, that both source and reservoir 
of the oil lie within the Pierre formation itself,—a condition of 
affairs quite natural, since, on the one hand, shales are in many re¬ 
gions the recognized place of origin of oil, and, on the other hand, 
the distribution of the materials of the Pierre would permit the accu¬ 
mulation of the entire product of the formation in the more open, 
porous, sandy bodies inclosed within the argillaceous portions. 

From the foregoing discussion it will be seen that no inferences 
can be drawn from the geology and well-data of the Florence field 
as to the existence of oil-bearing strata in other portions of the Rocky 
Mountain region and the adjacent prairies, even where underlain by 
beds of the same formation. That the Pierre carries oil in one 
locality has hardly the same force as an argument for its presence in 
another, as has the occurrence of workable coal in the Dakota for¬ 
mation, within a certain region, for its reappearance in another. In 
either case it is merely a recognized possibility that ought to be con¬ 
sidered. 


The Wells. 

On account of the relations which the companies of the Florence 
oil-field hold to each other in extent of development, in capital 
invested, and in business competition, a detailed discussion of the 
wells cannot be presented in this paper, but the general deductions 
given are based upon a most careful scrutiny of the individual 
records of almost the entire number of wells—a scrutiny rendered 
possible only through the great courtesy of the several company 
managers. 

Range of Flows in Strati graphical Horizon .—Within a distance of 
1155 feet beneath the summit of the oil-bearing zone, as provisionally 
established, no persistent succession of unproductive measures exists 
of a thickness greater than 100 feet. Midway between the depths of 
1155 feet and 1947 feet,—the latter being the horizon of the lowest 
present yield and the arbitrary base of the oil zone,—there are three 
more productive layers within a distance of 100 feet of one another, 
separated from those above and the one below by approximately 350 
feet of beds in which no flows have yet been struck. To obtain this * 
close succession of productive strata it is necessary to consider the 
field in its entirety. Within the area of a particular group of wells 


16 


THE FLORENCE OIL-FIELD, COLORADO. 


the intervals barren of oil are often increased to from 200 feet to 400 
feet, while in single wells they may become as great as 900 feet. 

In the examination and tabulation of the well records of the 
Florence field, two points of interest in connection with the flows 
were developed : the first, that for the field in general there are 
certain horizons in proximity to which oil has been found more fre¬ 
quently than in other parts of the vertical scale; the second, that 
wells yielding petroleum from depths of 400 feet or 500 feet below the 
summit of the oil-bearing zone, occur to the west of wells in which 
no flows are encountered at these depths and that, too, notwith¬ 
standing the continuation of their productive horizons considerably 
beyond the easternmost wells before their dip brings them to the 
surface. In explanation of the latter point, it is possible that the 
requisite sedimentary conditions for an oil-bearing rock do not 
exist over this portion of the field, but from an inspection of the 
sections and from the fact that oil is rarely found at a depth less 
than 800 feet or 900 feet beneath the surface, it would seem that 
some other cause, such as evaporation, may have had an influence 
toward this result. 

The vertical succession of oil-bearing strata in individual wells 
suggests another argument in favor of the origin of oil within the 
Pierre formation itself: in the event of its having been derived from 
a lower formation, it would have been largely taken up by the lower 
bodies of sands; the yields of the several horizons of a single well, 
however, afford no evidence of this condition, large flows occur¬ 
ring with equal frequency from either upper or lower oil-bearing 
bodies or from both. 

Relations between Wells .—No connection exists for the field in 
general between flows of the same horizon. Only within the nar¬ 
rower group limits have they been recognized, and here, between 
wells in close proximity to one another. But a single instance is 
known of an extent of influence greater than 100 feet or 200 feet, 
when a newly drilled well caused a marked and rapid decrease in the 
yield of another from the same horizon 1000 feet to the north-east, 
the dip being west. On the contrary, wells have been sunk within 
25 feet of another, without meeting a trace of oil at the horizon which 
in that first drilled afforded an excellent yield. In explanation of 
instances of this nature it is to be remarked that besides the nor- 
• Dial rapid changes in texture and composition of the oil “ sands,” 
such bodies, themselves containing considerable clay and surrounded 
by argillaceous shales, may, in the course of compression and flexing 


THE FLORENCE OIL-FIELD, COLORADO. 


17 


which they have here and there undergone in earlier times, have 
been rendered.locally more compact in texture or have become sud¬ 
denly and confusedly blended with the mass of shales, thus present¬ 
ing irregularities in arrangement of materials, all with influence upon 
the flow of the oil that may be contained within the limits of dis¬ 
turbance. Instances of this nature might also be accounted for by 
fissures, provided that direct evidence of their existence was found 
at the time of drilling. 

Percentage of Successful Wells .—Within the area of the oil-bear¬ 
ing zone of the Pierre formation, there are accounted for as having 
been drilled up to November 1, 1890, 82 productive and unproduc¬ 
tive wells. Of these, 16 per cent, have shown no trace of oil, and 
19J per cent, have shown traces less than an appreciable flow in 
from 1 to 6 horizons in a single well, or, in all, per cent, unpro¬ 
ductive wells. If to this be added 7 per cent.—the proportion of 
wells yielding less than 5 barrels and consequently below the present 
standard of economic pumping—the total percentage of unproduc¬ 
tive wells will be raised to 42J, leaving 57 J as the percentage of 
successful wells. 

The Yield .—When oil is struck in the Florence field, nearly all 
the productive wells show an abnormal rise, due to expansion 
of the gas contained in the oil; the oil ascends in the well from 
50 to 1000 feet, but upon being pumped settles within a few days 
to a level a little above that of the flow—a level which is main¬ 
tained until the well clogs or exhaustion begins to assert itself. 
The original yield of the wells, upon attaining the normal level, has 
ranged from 5 to 150 barrels per day ; several flows have yielded 
between 75 and 150 barrels, and a large number from 25 to 75 bar¬ 
rels. After a longer or shorter time a gradual and uniform decrease 
in production sets in, which, however, may extend over a period of 
several years before exhaustion is carried to the limit of economic 
pumping. Of the smaller wells, many may be temporarily ex¬ 
hausted by forced pumping, but after the lapse of a short interval 
of rest completely regain their former yield. Among the wells there 
are some that have maintained a constant flow during their entire 
existence to date—several years—while in a few instances actual 
increase to two or three times the original yield has occured. When¬ 
ever a yield has diminished through clogging of the well by accumu¬ 
lation of extraneous material, cleaning rarely fails to restore it to its 
normal amount. 

The areal distribution of the oil, as evidenced by the yield ot 
widely separated wells, is exceedingly uniform for the portion of the 


18 


THE FLORENCE OIL-FIELD, COLORADO. 


field thus far developed, both large and small flows being alike 
encountered in each of the various well-groups. 

The use of torpedoes has been tried in the Florence field in three 
instances, without success. Caving of the walls has resulted in each 
case, while within the range of action of the torpedo it is believed 
that on account of the clayey nature of the strata there has resulted 
a packing of materials, the reverse of the desired effect. 

The life of the wells of the Florence field is very variable : there 
are some of an age of over four years which are still good producers; 
many, over two, to date, among which are several of the largest; 
while of those whose duration has been less than a year there are 
comparatively few. The well of shortest life was one of six days of 
a total yield of 37 barrels. Examination of well records fails to 
disclose any relationship between the life and the size of the flow. 

Gas .—In the Florence field gas occurs chiefly in association with 
the oil, although small amounts have been encountered in the shales 
at horizons where no trace of oil was found. Associated with the 
oil, its quantity and rate of delivery is very variable. In some wells 
it is quickly exhausted while in others it remains persistent with a 
fair flow. There are instances in which it has manifested tremen¬ 
dous force upon being struck, carrying the oil beyond the derrick 
tops and for a while rendering difficult its control, afterwards quickly 
diminishing to an inappreciable amount. In other instances, after 
the first rush of gas, its flow has been maintained at a constant pres¬ 
sure and delivery during the entire life of the well. Neither pres¬ 
sure nor flow have ever been measured, but they are slight in com¬ 
parison with the eastern fields. The gas has been utilized only in 
lighting the immediate vicinity of the wells. 

Water .—This is met with in a few wells but it is an uncertain 
element. Its quantity is always small. Except in two instances it 
is fresh and has been found within 350 feet of the surface. The 
exceptions are salt, and were found in connection with the oil at a 
considerable depth. None of the water is artesian. No relation 
exists between the water and oil in the Florence field in the lines usu¬ 
ally attributed to them in eastern wells. Indeed, many of the wells 
are not only without any trace of water from the surface down but 
remain dry after exhaustion of the oil. There is nowhere any evi¬ 
dence of a water-pressure back of the oil. 

The Oil. 

Character , Production , Treatment , Products .— The crude product 
of the Florence field is of a light yellowish-green color, varies but 


THE FLORENCE OIL-FIELD, COLORADO. 


19 


slightly from an average gravity of 31° B., and is remarkably free 
from sand and other impurities, which in a month’s constant run 
amount to but about a gallon per still. 

The present production of the field is about about 2000 barrels 
per day, yielding upon distillation approximately one-third refined 
or illuminating-oil, a small amount only of lubricating-oil, and a 
residuum, which js stored. The production, however, is varied 
according to the demand for oil. The capacity of the field is con¬ 
siderably above 2000 barrels daily. For the twelve months to 
October 1, 1891, the total product amounted to about 300,000 
barrels of crude oil, from which was manufactured about 100,000 
barrels of illuminating oil and 5000 barrels of lubricating oil. 

The treatment of the crude oil as practiced by the United Oil 
Company, in the process of distillation, is the same as ordinarily em¬ 
ployed elsewhere. It passes from the wells to storage-tanks, and 
thence to the stills in charges of 400 barrels for the larger and 100 
barrels for the smaller. The run of the former is of 36 hours’ dura¬ 
tion, of the latter 20 hours. The yield from these is 40 per cent 
illuminating-oil, the gravity of the distillate being 44J° B., of the 
residuum, 22° B. From the stills the cooled distillate passes to the 
distillate storage tanks and thence to the agitators, where it is sub¬ 
jected to chemical treatment consisting of agitation with sulphuric 
acid (66° B.), and subsequent washing with a solution of caustic 
soda and water. Upon completion of the agitation it is run to the 
bleachers, settled, and from there distributed to the warehouse for 
packing or to the tank cars direct. The percentage of naphtha is 
insignificant. The burning-point is 130° F. 

The lubricating oil is derived from the residuum by passing the 
latter through a retort which removes the paraffine and raises the 
fire-test. This “stock’’ is compounded with neutral oil from the 
stills, which results in a product having a cold-test from 0° to 30° 
F., a flash-test of 350° F. and a gravity from 24° to 28° B. The 
residuum chills at 60° F. 

The quality of both illuminating and lubricating oils derived 
from Florence crude petroleum is said to be excellent; but for the 
latter product there is at present but slight demand, owing to eastern 
competition and freight-rates. 

The residuum from the manufacture of the above two classes of 
oil is stored in open reservoirs about the field, except such portion of 
it as is. required beneath the stills and boilers of the refineries and 
wells. In this use of it from 1200 to 1300 pounds (three to four 


20 


THE FLORENCE OIL-FIELD, COLORADO. 


barrels), are estimated to be the equivalent in effect of 2000 pounds 
of coal from $1.43 to $2 per ton at the mines. It is thrown into 
the fire-boxes in jets with the aid of super-heated steam, and not 
only affords a fire capable of instant regulation, but is of great 
economy in the saving of labor. 

The tanks for storage of the various products of the Florence 
field range in size from 5000 to 35,000 barrels. 

Discussion. 

In reply to the question, whether his investigations in the Flor¬ 
ence field had given any evidence concerning the origin of the 
petroleum, as animal or vegetable, Mr. Eldridge said : 

I omitted that topic from my paper, because I could throw no 
light upon it. The oil has not a strong odor. The formation con¬ 
tains animal remains in abundance, and through the fossiliferous 
lime-concretions there are many minute traces of plant-life, such as 
stems or particles of bark. As seems to be the case in most oil¬ 
bearing formations, nearly all evidence of life, except of animal 
life, has disappeared. The formation contains no coal in this local" 
ity, but coal is found in it at several localities in the West. 

F. H. Newell, Washington, D. C.: Mr. Eldridge’s paper throws 
some light on the subject of prospecting for oil. In the western 
fields we have had a curious succession of discoveries of oil. In 
Pennsylvania, West Virginia, and Ohio various theories have been 
projected or deduced, confining the occurrence of oil to certain con¬ 
ditions of structure, texture, position, etc.; but, on the other hand, 
the oil-operator himself, who backs up his opinion with his money, 
has treated with disregard, and even with contempt, all such studied 
generalizations, and has drilled his wells anywhere that fancy dic¬ 
tated. The result has been that he has spent immense sums, but he 
has discovered oil where the best experience would not have led 
those who knew most about it to advise him to drill. In the Flor¬ 
ence field we see the same thing. Here is a formation, the structure 
of which is quite the reverse of anything we had previously recog¬ 
nized as favorable, and near the upper limit of which, strange to say, 
oil has been found, nevertheless. Without discrediting the general 
conclusions which very able experts may have drawn, this case shows 
that in trying to anticipate the discovery of oil we are still com¬ 
pletely at a loss. The “ oil-man ,” with his persistence in drilling 
where he ought not to drill, has often brought valuable resources to 
light. 








